The following abbreviations are used in the present specification:
3GPPThird Generation Partnership ProjectCSCircuit SwitchedEDGEEnhanced Data Rates for GSM EvolutionEUTRANevolved UMTS Terrestrial Radio Access NetworkeNBEvolved Node BGSMGlobal System for Mobile CommunicationsLTELong Term EvolutionMMEMobility Management EntityPLMNPublic Land Mobile NetworkRATRadio Access TechnologyRFRadio FrequencySMSShort Messaging ServiceTAITracking Area IdentityUMTSUniversal Mobile Telecommunications SystemUSIMUniversal Subscriber Identity ModuleUSSDUnstructured Supplementary Service Data
In general, a wireless device on a first network may be using a first radio access technology but may need to switch to using a second, different radio access technology in order to access different services that are offered by that second radio access technology and not by the first radio access technology. The first and second radio access technologies may in general be provided by the same or equivalent networks or by different networks. However, once the device has reconnected to using that second radio access technology, there may not be a quick or effective way for the device to return to using the first radio access technology.
As a particular example, some mobile device services, such as voice calls and their associated supplementary services (e.g. call waiting and call forwarding services), USSD (Unstructured Supplementary Service Data) services and SMS (Short Messaging Services)), can only be achieved by using circuit switched technologies, at least in some applications or cases. However, in a particular case, radio access technologies (RATs) according to LTE (Long Term Evolution) standards do not comprise circuit switched technologies; LTE RATs are based entirely on packet switched technologies. Thus, in order to provide circuit switched services to devices using LTE RATs, such as EUTRAN, the devices must undergo a circuit switched (CS) fallback procedure, which reconnects the device to a “legacy” or other RAT that does support CS services. Such RATs include those using for example GSM (Global System for Mobile Communications), EDGE (Enhanced Data Rates for GSM Evolution) and UMTS (Universal Mobile Telecommunications System). Currently, LTE RATs are the only RATs that do not support circuit switched services. However in the future there may be other RATs developed that also do not support these services, and devices using these RATs would also need to reselect a different RAT to use CS services.
Once the CS service has been completed, the wireless device may be able to use normal cell reselection procedures to reselect the original LTE RAT. However, this is only the case if the wireless device is connected to the same (or “equivalent”) PLMN (Public Land Mobile Network) when using both the legacy and the LTE RATs, and in any event this can be a slow process, depending on for example signal strength and cell reselection parameters, which causes delays for the user. “Equivalent” PLMNs in this context are defined by 3GPP and discussed in more detail below, but for the moment, this can be summarised in that if a first PLMN is “equivalent” to a second PLMN, then a wireless device will treat both PLMNs as if they were the same.
Moreover, if a device has had to connect to a different PLMN as a result of the CS fallback procedure, there is currently no standard procedure to quickly reconnect the wireless device to its original PLMN. In theory, inter-PLMN redirection could take place, but this requires that the new PLMN know at least some information about the current PLMN so that the new PLMN can send a redirection command to the device. However, this requires cooperation between the current and new PLMNs, which in practice is unlikely to occur. Separately, in general, wireless devices undergo periodic so-called higher priority searches. The device has stored a list of PLMNs stored in a memory of the wireless device. These PLMNs are ranked in priority order, and the device will try to connect to the highest priority PLMN if it finds that it is not already connected to that PLMN. However, higher priority searches are typically infrequent, and, as such, the wireless device could remain connected to the legacy PLMN for a relatively long time, perhaps on the order of several minutes to 8 hours or more.